This invention relates to photoflash lamps and, more particularly, to flashlamps containing a combustible material which is ignited to produce actinic light.
A typical photoflash lamp comprises an hermetically sealed glass envelope containing a quantity of combustible metal, such as shredded zirconium or hafnium foil, and a combustion-supporting gas, such as oxygen, at a pressure well above one atmosphere. In lamps intended for battery operated flash systems, the envelope also includes an electrical ignition system comprising a tungsten filament supported on a pair of lead-in wires having a quantity of ignition paste on the inner ends thereof adjacent to the filament. This type of lamp is operated by the passage of an electrical current through the lead-in wires which incandesces the filament to ignite the ignition paste which in turn ignites the combustible metal in the envelope. In the case of percussive-type photoflash lamps, such as described in U.S. Pat. No. 3,535,063, a mechanical primer is sealed in one end of the lamp envelope. The primer may comprise a metal tube extending from the lamp envelope and a charge of fulminating material on an anvil wire supported in the tube. Operation of the percussive photoflash lamp is initiated by an impact onto the tube to cause deflagration of the fulminating material up through the tube to ignite the combustible metal disposed in the lamp envelopes.
Typically, the flashlamp envelope is comprised of G-1 type soft glass having a coefficient of thermal expansion within the range of 85 to 95 .times. 10.sup.-7 in./in./.degree. C. between 20.degree. C. and 300.degree. C., and the metal from which the primer tube is formed or the lead-in wires are made has a similar coefficient of thermal expansion so as to provide a match seal.
During lamp flashing, the glass envelope is subject to severe thermal shock due to hot globules of metal oxide and/or molten metal impinging on the walls of the lamp. As a result, cracks and crazes occur in the glass and, at higher internal pressures, containment failure becomes possible. In order to reinforce the glass envelope and improve its containment capability, it has been common practice to apply a protective lacquer coating on the lamp envelope by means of a dip process. To build up the desired coating thickness, the glass envelope is generally dipped a number of times into a lacquer solution containing a solvent and a selected resin, typically cellulose acetate. After each dip, the lamp is dried to evaporate the solvent and leave the desired coating of cellulose acetate, or whatever other plastic resin is employed.
In the continuing effort to improve light output, higher performance flashlamps have been developed which contain higher combustible fill weights per unit of internal envelope volume, along with higher fill gas pressures. In addition, the combustible material may be one of the hotter burning types, such as hafnium. Such lamps, upon flashing, appear to subject the glass envelopes to more intense thermal shock effects, and thus require stronger containment vessels. One approach to this problem has been to employ a hard glass envelope, such as the borosilicate glass envelope described in U.S. Pat. No. 3,506,385, along with a protective dip coating of cellulose acetate. More specifically, the patent describes an electrically ignitable lamp having in-leads of a metal alloy such as Rodar or Kovar secured by an internal expansion match seal in a glass envelope having a coefficient of thermal expansion in the range of 40 to 50 .times. 10.sup.-7 in./in./.degree. C. Type 7052 glass is mentioned as typical. The patent imposes a minimum of 40 .times. 10.sup.-7 in./in./.degree. C. on the coefficient of thermal expansion of the glass to assure the necessary match seal with the Rodar or Kovar in-leads. Further, it is theorized that glass in this thermal expansion range provides a more beneficial mode of fracture which results in a delay of crack time after flashing. More specifically, fracture of the glass is delayed to a time when the pressure in the lamp has been reduced to a point where containment is more readily assured. On the other hand, the use of hard glass incurs considerable added expense over the more commonly used soft glass due to both increased material cost and the need for special lead-in wires or primer tubes (e.g., Rodar or Kovar) to provide sealing compatibility with the low thermal expansion hard glass envelope. In addition, even though more resistant to thermal shock, hard glass envelopes do not eliminate fracture and often exhibit cracks and crazes upon lamp flashing.